Tetraethoxysilane (TEOS) has been used extensively as the source material for the chemical vapor deposition (CVD) of an SiO.sub.2 layer on a semiconductor wafer. U.S. Pat. No. 3,934,060, issued to Burt et al. on Jan. 20, 1976, describes on such prior art process.
In a typical prior art process nitrogen is used as the carrier gas which is passed over or bubbled through the TEOS which is held in a source container. The wafers to be covered with a deposited SiO.sub.2 film are laid in a furnace boat in a furnace tube into which the TEOS vapor entrained in the carrier gas is introduced and the wafers are heated to a selected temperature at which the TEOS pyrolytically decomposes. The range of temperatures less than approximately 960.degree. C. at which TEOS pyrolytically decomposes is well known. As the temperature of the wafers is reduced the decomposition rate and the film deposition rate decrease until they appear to stop. As a practical matter the range of temperatures typically employed ranges between 650.degree. C. and 950.degree. C. While the growth rate is lower for temperatures at the lower end of the range, these lower temperatures have the advantage of causing less thermal damage to the semiconductor circuit embedded in the underlying silicon wafer. These lower temperatures are also more compatible with metalization schemes using aluminum and its alloys.
U.S. Pat. No. 3,934,060, referred to above, describes an improvement in the geometry and deposition technique of the typical prior art process, which results in a more uniform coverage of the wafers by the SiO.sub.2 film.
It is also known that the growth rate of the SiO.sub.2 layer can be enhanced by using oxygen as a carrier gas (See U.S. Pat. No. 3,614,584, issued to Inoue on Oct. 19, 1971). The Inoue patent also describes a method for depositing TEOS using tetraethoxysilane as a source together with triisopropyl titanate in order to deposit a composite insulating layer of TiO.sub.2 -SiO.sub.2 where the TiO.sub.2 constitutes less than 0.02% by weight of the layer. Inoue employs the temperature range of 300.degree. C. to 500.degree. C. for the above process. Other methods using plasma deposition techniques are known for depositing an SiO.sub.2 based film on a video disk. For U.S. Pat. No. 4,282,268, issued Aug. 4, 1981 to Priestley et al., which is incorporated herein by reference, describes a method for forming an SiO.sub.2 layer on a video disk by introducing into an evacuated chamber a dielectric precursor having the formula ##STR1## where R.sub.1 is selected from the group H and --CH.sub.3, R.sub.2 and R.sub.3 are independently selected from the group consisting of H, --CH.sub.3, --OCH.sub.3 and --OC.sub.2 H.sub.5 and R.sub.4 is selected from the group consisting of --OCH.sub.3 and --OC.sub.2 H.sub.5. The SiO.sub.2 layer is deposited onto the disk using a glow discharge. This method has proven unsuitable for depositing an SiO.sub.2 based film on a semiconductor wafer because the deposited film has a poor dielectric quality, low density and is highly particulate, and because the deposited film has poor step coverage.